The architecture of a 3GPP Long Term Evolution, “LTE”, system is shown in FIG. 1, including an illustration of the X2 interfaces between base stations, referred to as “eNode Bs” or “eNBs” in the LTE standard, and the S1 interfaces between the eNBs and a Mobility Management Entity, “MME”, and Serving Gateway, “S-GW”. LTE is thus based on a rather flat architecture compared to 2G and 3G systems.
Each cell in an LTE network is served by an eNB, and handovers of wireless communication devices between network cells can be handled either by the MME via the S1 interface, or directly between the involved eNBs via the X2 interface. For details regarding the X2 interface and its associated protocols, see the 3GPP Technical Specification, TS 36.423 Rel-10 V10.3.0, for the X2 application protocol, “X2AP”, in Release 10. Note that the wireless communication devices supported by the illustrated network generally are associated with subscribers to the network, and may be referred to as user equipment, where “UE” denotes a singular item of such equipment and “UEs” denotes plural items.
In any given cellular network, there typically are areas with high traffic arising from high concentrations of users—here, “user” means the user and/or his or her associated UE, which is connected to the network and, at least intermittently, consumes network resources. Deploying additional capacity in areas of typically high user concentration is a desirable approach to ensuring higher user satisfaction in those areas.
The added capacity may derive from one or more additional “macro” base stations, which generally operate with significant power levels and offer radio service over a relatively large geographic area. Alternatively, the network operator may choose to supplement or augment the network by adding one or more lower-power base stations, e.g., “pico” base stations, which typically have a much smaller coverage area than the macro base stations, or “femto” base stations which may have an even smaller coverage area. Such deployments provide a concentrated “capacity boost” within the smaller coverage areas of the pico and femto base stations. Such capacity boosts may be used to provide higher data rates, for example.
Further, in the typical cellular network, there are areas of poor radio coverage. One approach to providing better coverage involves placing a pico or femto base station in the poor coverage area. These smaller base stations, more broadly referred to as low power nodes or “LPNs” may be, for example, Home NodeBs, “HNBs”, Home eNodeBs, “HeNBs”, Relay Nodes, “RNs”, etc. In any case, one argument for deploying one or more LPNs within the network is that the impact on the macro network can be minimized. That is, the macro base stations may be regarded as providing a macro layer of coverage, and the LPNs may be regarded as providing a pico or micro layer of coverage that overlays or extends the macro layer coverage.
Broadly, networks having such mixed deployments are referred to as Heterogeneous Networks or “HetNets”. FIG. 2 illustrates an example HetNet deployment scenario, where the coverage area of a macro cell includes one or more hotspots—areas of boosted capacity/coverage—provided via the deployment of LPNs. In the particular example provided in the diagram, there are a mix of LPN types, including pico cells, femto cells, and Relay Nodes, “RNs”. Further, although the figure shows clusters of femto cells, it is obvious that single cell deployments may also exist.
Mechanisms to reduce power—energy saving techniques—implemented in HetNets and, indeed, in communication networks in general, are environmentally friendly and they can reduce the capital expenditures (CAPEX) required of network operators. Energy savings are thus garnering increasing attention in standardization and commercial deployments. In the current 3GPP TS 36.300, it is suggested to turn off only those cells used for boosting capacity, but not those cells providing the basic network coverage.
FIG. 3 illustrates an example case, where one sees a macro cell serving as a basic-coverage cell. One further sees a pico-cell overlaid within the macro cell coverage area and serving as a capacity-boost cell. According to TS 36.300, the capacity-boost cell could be turned off for energy savings. In particular, the procedure specified in relevant part by TS 36.300 is as follows:
1. A macro eNB and a pico eNB exchange their load information via an X2AP message RESOURCE STATUS UPDATE.
2. According to the load information from macro eNB and itself, the pico eNB decides to turn off its transmission power and enter into a dormant mode. To do so, the pico eNB initializes its connected UEs to undertake handovers (HOs) to the basic-coverage cell. The pico eNB indicates to the macro eNB that the HO cause is “energy saving.”
3. The pico eNB turns off its capacity boost cell and indicates this event to the macro eNB via a Deactivation Indication Information Element, “IE”, in the X2AP: ENB CONFIGURATION UPDATE message. The macro eNB records the change.
4. When the load of the macro eNB's basic-coverage cell overfills, the macro eNB can reactivate the capacity-boost cell, to bring it out of its energy-saving mode, based on sending an X2AP:CELL ACTIVATION message to the pico eNB.
To maintain service continuity for UEs being handed over from the capacity-boost cell to the basic-coverage cell, the handovers initiated by the pico eNB in preparation for entering energy-saving operation must be conducted smoothly and must be subject to a low failure rate.
One issue related to achieving service continuity during such handovers relates to how the capacity-boost cell power is turned off. In particular, the question becomes how the pico eNB turns off transmission power in the capacity-boost cell gradually, so as to achieve a gradual decrease in capacity-boost cell coverage and thereby give time for gradually handing over the UEs supported by the capacity-boost cell. Improper settings of the power adjustment speed and extent of energy saving risks an increase in the handover failure rate. Handover failures negatively affect the user experience.
Alternatively, the basic-coverage cell targeted by these handovers might be unable to accept more incoming handovers, meaning that the process of powering down the capacity-boosting cell should be ceased. The procedures currently set forth in 3GPP TS 36.300 do not adequately detail these issues and are insufficient for addressing them.
The approach to reactivating a dormant capacity-boost cell represents another shortcoming of the currently contemplated procedures. According to current procedures, the macro eNB in a basic-coverage cell sends an indication to the pico eNB of the dormant capacity-boost cell, indicating that the capacity-boost cell should be reactivated. Here, the term “pico” is used somewhat generically, to cover pico cells, femto cells, and other smaller cells served by an LPN.
In any case, the reactivation process poses the problem of conducting reactivation in a manner that is sufficient for allowing the macro eNB to begin offloading UEs to the capacity-boost cell, while at the same time conducting the reactivation in an energy-efficient manner. If the adjustment speed (regarding adjustments step and period) and extent of energy saving are not set properly, the increased power might be unnecessary.